more explicit ref

Author: JakeFernandes98

.github/workflows/android-device-tests.yml

@@ -43,7 +43,7 @@ jobs:
         uses: walt-id/waltid-identity/.github/actions/checkout-repos@c95b601fc5095d5668eb8c906af992d8e00ed513
         with:
           token: ${{ secrets.GH_ACCESS_TOKEN || github.token }}
-          ref: ${{ github.head_ref || github.ref_name }}
+          ref: ${{ github.event.pull_request.head.ref || github.head_ref || github.ref_name }}
 
       - name: Configure gradle
         uses: walt-id/waltid-identity/.github/actions/gradle-setup-action@c95b601fc5095d5668eb8c906af992d8e00ed513